342 lines
12 KiB
Plaintext
342 lines
12 KiB
Plaintext
mkfs.btrfs(8)
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=============
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NAME
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----
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mkfs.btrfs - create a btrfs filesystem
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SYNOPSIS
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--------
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*mkfs.btrfs*
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$$[-A|--alloc-start <alloc-start>]$$
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$$[-b|--byte-count <byte-count>]$$
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$$[-d|--data <data-profile>]$$
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$$[-m|--metadata <metadata profile>]$$
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$$[-M|--mixed]$$
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$$[-l|--leafsize <leafsize>]$$
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$$[-n|--nodesize <nodesize>]$$
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$$[-s|--sectorsize <sectorsize>]$$
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$$[-L|--label <label>]$$
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$$[-K|--nodiscard]$$
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$$[-r|--rootdir <rootdir>]$$
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$$[-O|--features <feature1>[,<feature2>...]]$$
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$$[-U|--uuid <UUID>]$$
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$$[-f|--force]$$
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$$[-q|--quiet]$$
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$$[--help]$$
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$$[-V|--version]$$
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$$<device> [<device>...]$$
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DESCRIPTION
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-----------
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*mkfs.btrfs* is used to create the btrfs filesystem on a single or multiple
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devices. <device> is typically a block device but can be a file-backed image
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as well. Multiple devices are grouped by UUID of the filesystem.
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Before mounting such filesystem, the kernel module must know all the devices
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either via preceding execution of *btrfs device scan* or using the *device*
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mount option. See section *MULTIPLE DEVICES* for more details.
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OPTIONS
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-------
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*-A|--alloc-start <offset>*::
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(An option to help debugging chunk allocator.)
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Specify the (physical) offset from the start of the device at which allocations
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start. The default value is zero.
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*-b|--byte-count <size>*::
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Specify the size of the filesystem. If this option is not used,
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mkfs.btrfs uses the entire device space for the filesystem.
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*-d|--data <profile>*::
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Specify the profile for the data block groups. Valid values are 'raid0',
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'raid1', 'raid5', 'raid6', 'raid10' or 'single' or dup (case does not matter).
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+
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See 'DUP PROFILES ON A SINGLE DEVICE' for more.
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*-m|--metadata <profile>*::
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Specify the profile for the metadata block groups.
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Valid values are 'raid0', 'raid1', 'raid5', 'raid6', 'raid10', 'single' or
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'dup', (case does not matter).
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+
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A single device filesystem will default to 'DUP', unless a SSD is detected. Then
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it will default to 'single'. The detection is based on the value of
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`/sys/block/DEV/queue/rotational`, where 'DEV' is the short name of the device.
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+
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Note that the rotational status can be arbitrarily set by the underlying block
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device driver and may not reflect the true status (network block device, memory-backed
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SCSI devices etc). Use the options '--data/--metadata' to avoid confusion.
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+
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See 'DUP PROFILES ON A SINGLE DEVICE' for more details.
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*-M|--mixed*::
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Normally the data and metadata block groups are isolated. The 'mixed' mode
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will remove the isolation and store both types in the same block group type.
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This helps to utilize the free space regardless of the purpose and is suitable
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for small devices. The separate allocation of block groups leads to a situation
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where the space is reserved for the other block group type, is not available for
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allocation and can lead to ENOSPC state.
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+
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The recommended size for the mixed mode is for filesystems less than 1GiB. The
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soft recommendation is to use it for filesystems smaller than 5GiB. Thie mixed
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mode may lead to degraded performance on larger filesystems, but is otherwise
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usable, even on multiple devices.
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+
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The 'nodesize' and 'sectorsize' must be equal, and the block group types must
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match.
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+
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NOTE: versions up to 4.2.x forced the mixed mode for devices smaller than 1GiB.
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This has been removed in 4.3+ as it caused some usability issues.
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*-l|--leafsize <size>*::
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Alias for --nodesize. Deprecated.
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*-n|--nodesize <size>*::
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Specify the nodesize, the tree block size in which btrfs stores metadata. The
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default value is 16KiB (16384) or the page size, whichever is bigger. Must be a
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multiple of the sectorsize, but not larger than 64KiB (65536). Leafsize always
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equals nodesize and the options are aliases.
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+
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Smaller node size increases fragmentation but lead to higher b-trees which in
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turn leads to lower locking contention. Higher node sizes give better packing
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and less fragmentation at the cost of more expensive memory operations while
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updating the metadata blocks.
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+
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NOTE: versions up to 3.11 set the nodesize to 4k.
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*-s|--sectorsize <size>*::
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Specify the sectorsize, the minimum data block allocation unit.
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+
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The default value is the page size and is autodetected. If the sectorsize
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differs from the page size, the created filesystem may not be mountable by the
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kernel. Therefore it is not recommended to use this option unless you are going
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to mount it on a system with the appropriate page size.
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*-L|--label <string>*::
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Specify a label for the filesystem. The 'string' should be less than 256
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bytes and must not contain newline characters.
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*-K|--nodiscard*::
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Do not perform whole device TRIM operation on devices that are capable of that.
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*-r|--rootdir <rootdir>*::
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Populate the toplevel subvolume with files from 'rootdir'. This does not
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require root permissions and does not mount the filesystem.
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*-O|--features <feature1>[,<feature2>...]*::
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A list of filesystem features turned on at mkfs time. Not all features are
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supported by old kernels. To disable a feature, prefix it with '^'.
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+
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See section *FILESYSTEM FEATURES* for more details. To see all available
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features that mkfs.btrfs supports run:
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+
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+mkfs.btrfs -O list-all+
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*-f|--force*::
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Forcibly overwrite the block devices when an existing filesystem is detected.
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By default, mkfs.btrfs will utilize 'libblkid' to check for any known
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filesystem on the devices. Alternatively you can use the `wipefs` utility
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to clear the devices.
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*-q|--quiet*::
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Print only error or warning messages. Options --features or --help are unaffected.
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*-U|--uuid <UUID>*::
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Create the filesystem with the given 'UUID'. The UUID must not exist on any
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filesystem currently present.
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*-V|--version*::
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Print the *mkfs.btrfs* version and exit.
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*--help*::
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Print help.
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SIZE UNITS
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----------
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The default unit is 'byte'. All size parameters accept suffixes in the 1024
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base. The recognized suffixes are: 'k', 'm', 'g', 't', 'e', both uppercase and
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lowercase.
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MULTIPLE DEVICES
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----------------
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Before mounting a multiple device filesystem, the kernel module must know the
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association of the block devices that are attached to the filesystem UUID.
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There is typically no action needed from the user. On a system that utilizes a
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udev-like daemon, any new block device is automatically registered. The rules
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call *btrfs device scan*.
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The same command can be used to trigger the device scanning if the btrfs kernel
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module is reloaded (naturally all previous information about the device
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registration is lost).
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Another possibility is to use the mount options *device* to specify the list of
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devices to scan at the time of mount.
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# mount -o device=/dev/sdb,device=/dev/sdc /dev/sda /mnt
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NOTE: that this means only scanning, if the devices do not exist in the system,
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mount will fail anyway. This can happen on systems without initramfs/initrd and
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root partition created with RAID1/10/5/6 profiles. The mount action can happen
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before all block devices are discovered. The waiting is usually done on the
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initramfs/initrd systems.
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FILESYSTEM FEATURES
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-------------------
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*mixed-bg*::
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mixed data and metadata block groups, also set by option '--mixed'
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*extref*::
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(default since btrfs-progs 3.12, kernel support since 3.7)
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increased hardlink limit per file in a directory to 65536, older kernels
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supported a varying number of hardlinks depending on the sum of all file name
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sizes that can be stored into one metadata block
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*raid56*::
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extended format for RAID5/6, also enabled if raid5 or raid6 block groups
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are selected
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*skinny-metadata*::
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(default since btrfs-progs 3.18, kernel support since 3.10)
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reduced-size metadata for extent references, saves a few percent of metadata
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*no-holes*::
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improved representation of file extents where holes are not explicitly
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stored as an extent, saves a few percent of metadata if sparse files are used
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BLOCK GROUPS, CHUNKS, RAID
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--------------------------
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The highlevel organizational units of a filesystem are block groups of three types:
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data, metadata and system.
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*DATA*::
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store data blocks and nothing else
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*METADATA*::
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store internal metadata in b-trees, can store file data if they fit into the
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inline limit
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*SYSTEM*::
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store structures that describe the mapping between the physical devices and the
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linear logical space representing the filesystem
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Other terms commonly used:
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*block group*::
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*chunk*::
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a logical range of space of a given profile, stores data, metadata or both;
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sometimes the terms are used interchangably
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A typical size of metadata block group is 256MiB (filesystem smaller than
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50GiB) and 1GiB (larger than 50GiB), for data it's 1GiB. The system block group
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size is a few megabytes.
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*RAID*::
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a block group profile type that utilizes RAID-like features on multiple
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devices: striping, mirroring, parity
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*profile*::
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when used in connection with block groups refers to the allocation strategy
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and constraints, see the section 'PROFILES' for more details
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PROFILES
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--------
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There are the following block group types available:
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[ cols="^,^,^,^,^",width="60%" ]
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|=============================================================
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.2+^.<h| Profile 3+^.^h| Redundancy .2+^.<h| Min/max devices
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^.^h| Copies ^.^h| Parity ^.<h| Striping
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| single | 1 | | | 1/any
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| DUP | 2 / 1 device | | | 1/1 ^(see note)^
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| RAID0 | | | 1 to N | 2/any
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| RAID1 | 2 | | | 2/any
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| RAID10 | 2 | | 1 to N | 4/any
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| RAID5 | 1 | 1 | 2 to N - 1 | 2/any
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| RAID6 | 1 | 2 | 3 to N - 2 | 3/any
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|=============================================================
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'Note:' DUP may exist on more than 1 device if it starts on a single device and
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another one is added, but *mkfs.btrfs* will not let you create DUP on multiple
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devices.
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DUP PROFILES ON A SINGLE DEVICE
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-------------------------------
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The mkfs utility will let the user create a filesystem with profiles that write
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the logical blocks to 2 physical locations. Whether there are really 2
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physical copies highly depends on the underlying device type.
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For example, a SSD drive can remap the blocks internally to a single copy thus
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deduplicating them. This negates the purpose of increased redunancy and just
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wastes space.
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The duplicated data/metadata may still be useful to statistically improve the
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chances on a device that might perform some internal optimizations. The actual
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details are not usually disclosed by vendors. As another example, the widely
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used USB flash or SD cards use a translation layer. The data lifetime may
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be affected by frequent plugging. The memory cells could get damaged, hopefully
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not destroying both copies of particular data.
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The traditional rotational hard drives usually fail at the sector level.
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In any case, a device that starts to misbehave and repairs from the DUP copy
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should be replaced! *DUP is not backup*.
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KNOWN ISSUES
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------------
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**SMALL FILESYSTEMS AND LARGE NODESIZE**
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The combination of small filesystem size and large nodesize is not recommended
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in general and can lead to various ENOSPC-related issues during mount time or runtime.
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Since mixed block group creation is optional, we allow small
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filesystem instances with differing values for 'sectorsize' and 'nodesize'
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to be created and could end up in the following situation:
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# mkfs.btrfs -f -n 65536 /dev/loop0
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btrfs-progs v3.19-rc2-405-g976307c
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See http://btrfs.wiki.kernel.org for more information.
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Performing full device TRIM (512.00MiB) ...
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Label: (null)
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UUID: 49fab72e-0c8b-466b-a3ca-d1bfe56475f0
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Node size: 65536
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Sector size: 4096
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Filesystem size: 512.00MiB
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Block group profiles:
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Data: single 8.00MiB
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Metadata: DUP 40.00MiB
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System: DUP 12.00MiB
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SSD detected: no
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Incompat features: extref, skinny-metadata
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Number of devices: 1
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Devices:
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ID SIZE PATH
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1 512.00MiB /dev/loop0
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# mount /dev/loop0 /mnt/
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mount: mount /dev/loop0 on /mnt failed: No space left on device
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The ENOSPC occurs during the creation of the UUID tree. This is caused
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by large metadata blocks and space reservation strategy that allocates more
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than can fit into the filesystem.
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AVAILABILITY
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------------
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*mkfs.btrfs* is part of btrfs-progs.
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Please refer to the btrfs wiki http://btrfs.wiki.kernel.org for
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further details.
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SEE ALSO
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--------
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`btrfs`(8), `wipefs`(8)
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